Western Equine Encephalitis Research Articles

Venezuelan equine encephalomyelitis

A virus was isolated from the brain of an infected horse and was shown to be different from the two North American viral ence-phalitides, eastern and western equine encephalomyelitis; this new agent was called Venezuelan equine encephalomyelitis (VEE) virus. Historically, awareness of VEE virus activity was predicted upon the dramatic, devastating outbreaks of clinical disease in equines and humans of northern South America. The equine virulent epizootic VEE virus variants I-AB and I-C cause severe, often fatal infections in equines and humans. Clinical disease in equines is the major indicator of epizootic VEE virus activity. There are numerous areas of needed research in the epidemiology of VEE, many of which address the origin of epizootics and the interepizootic behavior or persistence of epizootic VEE virus. There are numerous areas of needed research in the epidemiology of VEE, many of which address the origin of epizootics and the interepizootic behavior or persistence of epizootic VEE virus.

Antibodies to arthropod-borne encephalitis viruses in small mammals from southern Florida.

From 1987 through 1991, blood samples were collected from 10 species of small mammals in Indian River Country, Florida (USA). Sera from 1,347 animals were analyzed for hemagglutination-inhibition (HI) antibody to St. Louis encephalitis (SLE) and eastern equine encephalitis (EEE) viruses. Of these, 75 (5.6%) were positive for HI antibody to SLE virus and 121 (9.0%) were positive for EEE antibody. Sera from five mammalian species were tested for neutralizing (NT) antibody to SLE, EEE, Highlands J (HJ a member of the western equine encephalitis virus complex), or Everglades (EVE, a member of the Venezuelan equine encephalitis complex) viruses. By serum neutralization tests, 26 (46%) of 57 had SLE antibodies, 14 (24%) of 58 had EEE antibodies, two (3.2%) of 63 had HJ antibodies, and 9 (14%) of 63 had EVE antibodies. One Sigmodon hispidus and one Peromyscus gossypinus had NT antibodies both to EEE and HJ viruses. Blood samples from 512 mammals were tested for virus. Isolations of one EVE virus and two unidentified arenaviruses were made from P. gossypinus and one EVE virus isolate was made from a S. hispidus.

Seasonal variation in the vector competence of Culex tarsalis (Diptera:Culicidae) from the Coachella Valley of California for western equine encephalomyelitis and St. Louis encephalitis viruses.

The vector competence of Culex tarsalis Coquillett from the Coachella Valley of California for western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses was monitored monthly from February to November 1993. The concentration of WEE virus required to infect 50% of the females increased during summer coincidentally with ambient temperature and was highest during July. Transmission rates of WEE virus were high during March, low during May-June, and high again during July-September. Females expressed both mesenteronal escape and salivary gland barriers limiting WEE virus dissemination and transmission rates, respectively. SLE virus infection and dissemination rates did not vary among months, but transmission rates, were highest during July-September. Although infection rates with SLE virus were moderate, most infected females developed disseminated infections. Salivary gland infection or escape barriers prevented SLE virus transmission in 16-100% of infected females.

Arboviral Disease—United States, 1994

Arboviruses are mosquitoborne and tickborne agents that persist in nature in complex cycles involving birds and mammals, including humans. Characteristics of arboviral infection include fever, headache, encephalitis, and sometimes death. In 1994, health departments in 20 states reported 100 presumptive or confirmed human cases of arboviral disease to CDC. Of these, 76 were California (CAL) serogroup encephalitis; 20, St. Louis encephalitis (SLE); two, western equine encephalomyelitis (WEE); one, eastern equine encephalomyelitis (EEE); and one, Powassan encephalitis (POW). This report summarizes information about arboviral disease in the United States during 1994.

Daily pattern of flight activity of Aedes albifasciatus in central Argentina.

Aedes albifasciatus is an important common mosquito in Central Argentina. It is a confirmed vector of the Western Equine Encephalitis and is responsible for loss of milk production in dairy cattle during peak populations. This paper reports the flight activity pattern of Ae. albifasciatus for different seasons, in the southern coast of the Mar Chiquita Lake (central Argentina). Data were collected by sampling two sites between 1992 and 1993 with CDC traps and human-bait captures. Adult mosquito population density, estimated by CDC trapping and human-bait, were highly correlated. However, when compared to other species, the proportion of Ae. albifasciatus was higher in human-bait collections. Adult female populations were active only when temperature were higher than 6 degrees C. Two daily biting peaks were observed (dusk and dawn) during the spring, summer and autumn, and only one peak during winter (around 15:00 pm). Adult abundance was significantly correlated (R2 = 0.71; p < 0.01) with temperature and illumination.

Arbovirus Infections

Arbovirus infections are viral diseases transmitted by insect vectors—that is, they are arthropod borne. Clinically significant arboviruses in the United States include western equine encephalitis virus (WEE), eastern equine encephalitis virus (EEE), St. Louis encephalitis virus (SLE), the La Crosse (LAC) strain of the California encephalitis group, Powassan virus (POW), and Colorado tick fever virus (CTF). The four encephalitis viruses are spread by mosquitoes, whereas POW and CTF are tick borne. Not surprisingly, given that transmission depends on mosquito and tick vectors, arbovirus infections are most common in the US during the late spring and summer months. The geographic distribution of these viruses (see Table) is helpful to diagnosis because their clinical manifestations are not distinctive enough in individual patients to be diagnostic.

The biology of Aedes (Ochlerotatus) albifasciatus Macquart, 1838 (Diptera: Culicidae) in Central Argentina

Aedes albifasciatus is a flood water mosquito ocurring in the southern countries of South America. It is a competent vector of the Western Equine Encephalitis (WEE) and causes important losses on milk and beef production in central Argentina. Field work was carried out from December 1990 to March 1993, on a monthly basis during the dry season and biweekly during the rainy season. Larvae were collected using the 'dipping' technique and females with CDC traps baited with CO2. Field collected larvae were used to build laboratory cohorts, from which basic population parameters were estimated. Eggs survived up to six months on dry soil, although there was a linear decrease of viability with time. At 23oC, larval development time was around nine days, and all adults emerged within one week. The estimation of larval development in the laboratory seems to be very near the development on the field, as larvae have been collected on average eight days after a rainfall. Egg to adult survival was 83%, with the highest mortality on fourth larval instar (6%). In the laboratory studies, sex proportion among the adults was 1:1, females lived longer than males (median 13 and five days, respectively), and adult survival pattern showed a constant number of individuals dying per unit time. Field collected females layed an average of 84 eggs per batch, and completing up to five gonotrophic cycles, suggesting an estimated survival of up to 35-50 days.

Landscape ecology of arboviruses in southern California: temporal and spatial patterns of vector and virus activity in Coachella Valley, 1990-1992.

Consistent temporal and spatial patterns in the activity of Culex tarsalis Coquillett and western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were delineated that were useful in developing a stratified surveillance program. Vernal increases in Cx. tarsalis abundance typically were associated with flooding of saline marshes along the north shore of the Salton Sea and were followed 6-8 wk later by the onset of WEE and SLE virus activity. Viruses then spread to managed marsh (duck club) and agricultural habitats in the Whitewater Channel flood plain and, depending upon the intensity of amplification, to agricultural and residential areas in the more elevated northwestern portion of the valley. Mean annual Cx. tarsalis abundance was correlated inversely with elevation and distance from the Salton Sea. Abundance was greatest at managed marsh habitats. Although spatially correlated with vector abundance among sites, virus transmission rates to sentinel chickens were asynchronous temporally with vector abundance. Seroconversion rates were related to flock location but not flock size (10 versus 20 chickens). Human cases were not detected during the study period, despite elevated transmission rates of both WEE and SLE viruses to sentinel chickens positioned in peridomestic habitats.

Bionomics of Culex tarsalis (Diptera: Culicidae) in relation to arbovirus transmission in southeastern California.

Population dynamics and bionomics of host-seeking Culex tarsalis Coquillett were studied in the Imperial and Coachella valleys of California during periods in 1991 and 1992 when western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses were transmitted to sentinel chickens. Female abundance was greatest during the spring and fall, before and after most virus transmission occurred and was not correlated with temperature, humidity, or rainfall. Parity rates were highest during late summer when virus activity peaked and were lowest during December when females may enter a short-term reproductive diapause. Although most likely underestimated, the proportion of older multiparous females were collected at a consistent, but low level throughout the year. Changes in the parity rate seemed to be influenced primarily by the proportions of 1-parous females. Survivorship estimated from the parity rate (adjusted to account for autogeny) was highest in winter; however, the proportion of females surviving to potentially transmit either WEE or SLE virus was highest in summer and early fall. Wing length decreased in summer as an inverse correlate of temperature and increased as a function of female age, implying that larger females lived longest. However, autogenous females were larger than anautogenous females at emergence and only parous autogenous females were collected host seeking, thereby confounding the relationship between size and age. The proportion of females testing positive for fructose was greatest during winter and lowest during summer, perhaps affecting survivorship and blood-feeding avidity. The vector competence (infection, dissemination and transmission rates, and ID50) of females collected host seeking or emerging from field-collected pupae for WEE or SLE viruses remained similar over time, even though the wing length of females used in these experiments differed among samples. We conclude that in nature virus transmission progressed efficiently during midsummer because elevated temperatures shortened the extrinsic incubation period without markedly decreasing survivorship resulting in an increased proportion of females surviving extrinsic incubation to become infective.

Landscape Ecology of Arboviruses in Southern California: Patterns in the Epizootic Dissemination of Western Equine Encephalomyelitis and St. Louis Encephalitis Viruses in Coachella Valley, 1991–1992

Temporal and spatial patterns in the initiation and dissemination of western equine encephalomyelitis and St. Louis encephalitis virus activity in Coachella Valley during 1991 and 1992 were detected by testing pools of host-seeking Culex tarsalis Coquillett for virus infection and sentinel chickens for seroconversions. Both viruses repeatedly were detected first at a salt marsh adjacent to the Salton Sea in the southeastern corner of the study area and then disseminated to the northwest to freshwater marsh, agricultural, and residential habitats. Virus dissemination was relatively slow (< 1 km/d) and may have been accomplished by dispersive host-seeking mosquitoes. Repeated early-season recovery of virus activity indicated that both viruses may persist interseasonally in salt marsh habitat.

Aura Virus Is a New World Representative of Sindbis-like Viruses

Aura virus is an alphavirus present in Brazil and Argentina that is serologically related to Sindbis virus (present throughout the Old World) and to Western equine encephalitis (WEE) virus (present in the Americas). We have previously shown that WEE is a recombinant virus whose glycoproteins and part of whose 3′ nontranslated region (NTR) are derived from a Sindbis-like virus, but the remainder of whose genome is derived from Eastern equine encephalitis (EEE) virus. We show here that Aura virus is a Sindbis-like virus that shares considerable organizational and sequence identity with Sindbis virus. Certain nucleotide sequence elements present in Aura RNA that are believed to function as promoters are almost identical to their Sindbis counterparts, repeated elements in the 3′ nontranslated region are shared with Sindbis virus, and important antigenic epitopes are conserved between the two viruses. Despite their close relationship, the two viruses have diverged significantly, sharing 73% amino acid sequence identity in the nonstructural proteins and 62% identity in the structural proteins. This is about the same as the identities between EEE and Venezuelan equine encephalitis virus, whose promoter elements, 3′ NTRs, and antigenic epitopes have diverged more radically, such that these two viruses are considered to belong to different subgroups. Importantly, the glycoproteins of WEE are more closely related to those of Sindbis than to those of Aura virus. From this we propose that an ancestral Sindbis-like virus present in the Americas (probably South America) diverged 1000-2000 years ago into a lineage that gave rise to Aura virus and a lineage that gave rise to Sindbis virus and to the Sindbis-like parent of WEE. At some time after this divergence, a Sindbis-like virus belonging to the latter lineage was transferred to the Old World where it gave rise to Sindbis viruses distributed throughout the Old World, and in a separate event a Sindbis-like virus belonging to the same lineage underwent recombination with EEE to give rise to WEE.

Sensitive and specific colorimetric dot assay to detect eastern equine encephalomyelitis viral RNA in mosquitoes (Diptera: Culicidae) after polymerase chain reaction amplification.

A sensitive and specific colorimetric dot assay following polymerase chain reaction (PCR) method has been developed to detect 0.1 pg of eastern equine encephalomyelitis viral (EEEV) RNA. The assay is 250-fold more sensitive than analysis by electrophoresis and is based on converting a 291-nucleotide sequence of the viral coat protein amino terminus into a double-stranded DNA (dsDNA) and amplifying the DNA using a specific primer pair and PCR. The amplified complementary DNA (cDNA) is denatured adsorbed onto a nylon strip, baked, and detected with a digoxigenin-labeled probe. Dots with viral cDNA are stained dark red, whereas controls do not stain or stain lightly. The assay is very specific and sensitive and detects only EEEV. RNA of Venezuelan equine encephalitis, St. Louis encephalitis, Keystone, Flanders, Tensaw, and western equine encephalitis viruses were not detected. EEEV (Ten Broeck) RNA was detected at the 10-ng level, indicating that the prototype we used may have different nucleotides in the region where the primer pair binds. The PCR amplified EEEV cDNA that was 92% homologous to the consensus sequence of EEEV. The detection of EEEV in the liver of an infected Emu bird and in field-collected mosquitoes from Florida and Massachusetts that were analyzed concurrently as blind samples by tissue culture plaque assay and by PCR dot analysis proved that the assay is sensitive and can be used to detect infected mosquitoes. The assay can detect at least 1 infected mosquito in a pool of 1,000 uninfected mosquitoes.

Serologic survey of neotropical bats in Guatemala for virus antibodies.

Neotropical bats were collected from different life zones in Guatemala in 1983 and 1984 to determine the presence and distribution of antibody against 10 viruses. Bats were collected with mist nets at 13 sites in eight departments and 332 serum specimens were obtained for testing for neutralizing (N) antibody by the plaque-reduction neutralization test. Eighty-seven (26%) of the 332 bats from 16 (38%) of 42 bat species sampled were serologically positive for five of six arboviruses and for two other viruses tested. Antibodies against Venezuelan equine encephalitis (VEE) variant I-A/B, eastern equine encephalitis, western equine encephalitis, St. Louis encephalitis, vesicular stomatitis, Tacaribe, and Rio Bravo viruses were detected in resident species of bats. However, N antibodies against the enzootic strain of VEE (Mena II, variant I-E) or Nepuyo viruses were not detected.

Nematocera (Ceratopogonidae, Psychodidae, Simuliidae and Culicidae) and control methods.

The biology, veterinary importance and control of certain Nematocera are described and discussed. Culicoides spp. (family Ceratopogonidae) transmit the arboviruses of bluetongue (BT), African horse sickness (AHS), bovine ephemeral fever (BEF) and Akabane. Some other arboviruses have been isolated from these species, while fowl pox has been transmitted experimentally by Culicoides. These insects are vectors of the parasitic protozoans Leucocytozoon caulleryi and Haemoproteus nettionis, and the parasitic nematodes Onchocerca gutturosa, O. gibsoni and O. cervicalis. They also cause recurrent summer hypersensitivity in horses, ponies, donkeys, cattle and sheep. Farm animals can die as a result of mass attack by Simulium spp., which are also vectors of Leucocytozoon simondi, L. smithi and the filariae O. gutturosa, O. linealis and O. ochengi. Venezuelan equine encephalomyelitis (VEE) and Rift Valley fever (RVF) have been isolated from simuliids, and vesicular stomatitis virus New Jersey strain has been replicated in Simulium vittatum. Simuliids are well known as vectors of O. volvulus, the cause of human onchocercosis (river blindness). The family Psychodidae includes the genera Phlebotomus and Lutzomyia (subfamily Phlebotominae), vectors of Leishmania spp. in humans, dogs and other mammals. Vesicular stomatitis virus Indiana strain has been regularly isolated from phlebotomine sandflies. Mass attack by mosquitoes can also prove fatal to farm animals. Mosquitoes are vectors of the viruses of Akabane, BEF, RVF, Japanese encephalitis, VEE, western equine encephalomyelitis, eastern equine encephalomyelitis and west Nile meningoencephalitis, secondary vectors of AHS and suspected vectors of Israel turkey meningoencephalitis. The viruses of hog cholera, fowl pox and reticuloendotheliosis, the rickettsiae Eperythrozoon ovis and E. suis, and the bacterium Borrelia anserina are mechanically transmitted by mosquitoes. These insects also induce allergic dermatitis in horses. They transmit several filarial worms of both animals and humans, and are of great medical importance as vectors of major human diseases, including malaria, yellow fever, dengue fever and many more diseases caused by arboviruses.

Western equine encephalitis virus in emus in Oklahoma.

Economic Community, Annex 2-a and 2-b. USDA, Washing30. Work TM, Jessup DA, Sawyer MM: 1992, Experimental blueton, DC. tongue and epizootic hemorrhagic disease virus infection in Cal29. Wade-Evans AM, Mertens PPC, Bostock CJ: 1991, Developifornia black-tailed deer. J Wildl Dis 28:623-628. ment of polymerase chain reaction for the identification of bluetongue virus in tissue samples. J Virol Methods 30: 15-24.

Susceptibility of the Sf9 Insect Cell Line to Infection with Adventitious Viruses

Abstract. Sf9, the insect cell line commonly used for gene expression by recombinant baculovirus (BV), can be infected by St. Louis encephalitis (SLE) virus, a flavivirus, resulting in a persistent, productive, and cytopathic infection, while retaining the ability to be infected with a recombinant baculovirus (rBV). We now demonstrate using double immunofluorescence that single cells are dually infected with SLE virus and rBV. Fourteen additional viruses including additional flaviviruses, other arbovirus classes, vesicular stomatitis virus (VSV), and herpes simplex virus, type 1 (HSV-1) failed to produce a cytopathic effect (CPE) in Sf9 cells. Plaque assays indicated infectious virus was present for several weeks post-inoculation for Yellow fever (YF), Dengue types 1 and 2 (DEN-1 and DEN-2), Gumbo limbo (GL), Eastern equine encephalomyelitis virus (EEE), Western equine encephalomyelitis virus (WEE), HSV-1, and VSV viruses. For HSV-1, GL, EEE, WEE and VSV, but not for YF, DEN-1 or DEN-2 viruses, this could be attributed solely to survival in the Sf9 cell culture media. Of the 14 viruses tested, only HSV-1 could be detected after 2 weeks in serum-free media. The data indicate that several viruses which are pathogenic for humans are stable for long periods of time at 27°C in the serum-containing media used for cultivation of Sf9 cells. YF, DEN-1 and DEN-2 viruses may replicate in Sf9 cells at extremely low levels. This suggests that adventitious agents which do not produce obvious CPE or interfere with rBV infection or recombinant protein expression could contaminate Sf9 cell cultures or media.

An epizootic attributable to western equine encephalitis virus infection in emus in Texas

An epizootic attributable to western equine encephalitis virus infection in emus in Texas

Surveillance of arbovirus infections in the Atlantic Forest Region, State of São Paulo, Brazil. I. Detection of hemagglutination-inhibiting antibodies in wild birds between 1978 and 1990.

We report data related to arbovirus antibodies detected in wild birds periodically captured from January 1978 to December 1990 in the counties of Salesópolis (Casa Grande Station), Itapetininga and Ribeira Valley, considering the different capture environments. Plasmas were examined using hemagglutination-inhibition (HI) tests. Only monotypic reactions were considered, except for two heterotypic reactions in which a significant difference in titer was observed for a determined virus of the same antigenic group. Among a total of 39,911 birds, 269 birds (0.7%) belonging to 66 species and 22 families were found to have a monotypic reaction for Eastern equine encephalitis (EEE), Venezuelan equine encephalitis (VEE), Western equine encephalitis (WEE), Ilheus (ILH), Rocio (ROC), St. Louis encephalitis (SLE), SP An 71686, or Caraparu (CAR) viruses. Analysis of the data provided information of epidemiologic interest with respect to these agents. Birds with positive serology were distributed among different habitats, with a predominance of unforested habitats. The greatest diversity of positive reactions was observed among species which concentrate in culture fields.

Potential effect of global warming on mosquito-borne arboviruses.

If global warming occurs in California, daily mean temperatures may increase by 3 to 5 degrees C, precipitation patterns will change, and sea level may rise 1 m. Studies were done on effect of temperature changes on survival of Culex tarsalis Coquillett, the primary vector of western equine encephalomyelitis (WEE) and St. Louis encephalitis (SLE) viruses, in two regions where temperatures differed by 5 degrees C. Daily mortality of adult vectors increased by 1% for each 1 degree C increase in temperature. At 25 degrees C, only 5% of Cx. tarsalis survived for 8 or more days, the time required for extrinsic incubation of these viruses. Extrinsic incubation times for these viruses shortened when temperatures were increased from 18 to 25 degrees C. WEE virus infection was modulated and transmission decreased at 32 degrees C. If temperatures in the warmer region increase by 5 degrees C, WEE virus may disappear and SLE virus would persist. In the cooler region, a 5 degrees C increase would decrease vector survivorship and virus activity in midsummer. In North America, epidemics of WEE have prevailed above a 21 degrees C isotherm and those of SLE below this isotherm. With global warming, epidemics of these viruses could extend into currently unreceptive northern areas. WEE virus would disappear from more southern regions. Geographic distribution of vector, human, and animal populations could be altered. North America could become more receptive to invasion by tropical vectors and diseases.

Natural vertical transmission of western equine encephalomyelitis virus in mosquitoes.

The mechanism by which western equine encephalomyelitis (WEE) virus and other mosquito-borne alphaviruses (Togaviridae) survive during periods of vector inactivity is unknown. Recently, three strains of WEE virus were isolated from adult Aedes dorsalis collected as larvae from a salt marsh in a coastal region of California. This provides evidence of vertical transmission of WEE virus in mosquitoes in nature. Vertical transmission in Ae. dorsalis and closely related mosquito species may be an important mechanism for the maintenance of WEE virus in temperate regions in North America where horizontal transmission of the virus is seasonal.